The invention relates to an antenna diversity receiving system for mobile reception of television signals as stated in the introductory part of claim 1.
Such antenna diversity receiving systems are preferably used to improve television reception in private motor cars.
At the 37th IEEE Vehicular Technology Conference, Tampa, Fla. Jun. 1-3, 1987, an antenna diversity receiving system was proposed. In this system 4 antenna signals are applied to a switching unit. In a comparator inserted before the television receiver, the antenna signals are compared with each other during the field blanking interval and, by means of switching diodes for avoiding picture flutter, the better antenna signal is switched to during the field blanking interval.
However, because the evaluation of the antenna signal level and the switching-over to the picture synchronization is included, a relatively slowly operating diversity system is provided which is capable of presenting a better antenna signal to the television receiver in the best case within a 20 ms time interval. With a receiving frequency of, for example, 800 MHz, already at a vehicle speed of 10 km/h a stretch of road corresponding to about 1.5 times the wavelength is covered during these 20 ms. When such antenna diversity receiving systems are used in motor cars it is very much a matter of the dynamic behaviour of the diversity system, because on the basis of the movements of the vehicle the antenna voltages change continuously and all the more so the shorter the wavelength of the signals to be received is. Therefore, it is necessary to verify the signal quality at very brief time intervals.
Additionally, the following further aspects are to be considered.
Since the electromagnetic waves are superimposed, it may also happen that even in a stationary vehicle no television reception is possible when at the receiving point the waves incident on the receiving antenna from various directions are accidentally cancelled.
The superpositioning of electromagnetic waves with large separate delays may also lead to the fact that on the television screen (monitor) so-called ghost images become visible which affect reception.
As a result of the television signal channel bandwidth which is large compared with that of radio, frequency-selective fading may additionally occur so that, for example, the chrominance subcarrier is cancelled and only a black-and-white picture is received instead of a colour picture transmitted by the transmitter.
As a result of the movement of the vehicle and the directivity of the reception this may further lead to intermodulation interference and adjacent channel interference.